1. Field of the Invention
The invention relates to silicon crystals useful in solar cells and somewhat more particularly to a method and apparatus for producing large surface plate or disc-shaped silicon crystals with columnar structures useful for further processing into solar cells.
2. Prior Art
When solar cells are produced from silicon, the most economically available silicon material should be utilized since the requirement for such component elements relative to crystal quality are not as high as those typically required in producing semiconductor component elements useful in integrated circuits.
Accordingly, it is desirable to find a means of producing silicon crystals which is simple and economical and does not involve excessive loss of materials. Further, such material-wasting and time-consuming steps as, for example, sawing a silicon bar produced by traditional crystal-growth methods, into crystal discs as well as lapping and polishing of such disc surfaces should be eliminated.
German Offenlegungsschrift (hereinafter DT-OS) No. 25 08 803 suggests that plate or disc-shaped silicon crystals having a columnar structure are useful as base materials for producing solar cells whereby an efficiency of more than 10% can be achieved. The process of producing such silicon crystals set forth in this DT-OS generally comprises providing a melt composed of a pre-purified polycrystalline silicon, feeding or casting such melt into a cooled graphite mold of suitable shape and allowing the so-cast melt to solidify in the mold over a temperature gradient. After solidification, the bar-shaped silicon crystals have a columnar structure formed in a direction of the shortest axis of the monocrystalline crystal areas, with crystallographic preferred orientation and exhibit semiconductive properties.
In the manufacture of solar cells from such bar-shaped silicon crystals, crystal discs of about 100.times.100 mm.sup.2 and a thickness of about 500 .mu.m are sawed from the bars produced in the above described manner with diamond saws conventional in semiconductor technology. Solar cells produced in accordance with known techniques from such discs have an efficiency which fluctuates between about 8.2% at a cell edge, up to 10.5% at the center of the cell. The efficiency thus achieved very nearly matches that of solar cells produced from monocrystalline silicon, which exhibits an efficiency of about 12% to 14%. However, the process described by the above-referenced DT-OS requires a sawing process to divide the bars into discs and such sawing cannot be omitted. Further, the size of the bars is determined by the size of the casting mold required for their manufacturing.
Another process for producing economical silicon is suggested in Electronics, page 108, Apr. 4, 1974. With this process, a polycrystalline silicon band having a length of at least 1 meter is produced by casting a silicon melt onto a cooled traveling carrier body composed of molybdenum or onto a traveling band coated with a silicon nitride layer so that the polycrystalline band is produced in a manner somewhat similar to assembly-line principles. However, the resultant silicon material does not have columnar structure so that solar cells produced therefrom exhibit an efficiency of less than about 5%.
Co-pending U.S. patent application Ser. No. 092,636 and Ser. No. 092,637 describe methods and apparatuses for producing plate or disc-shaped silicon bodies with columnar structure. In these applications, the columnar structure in the silicon crystal is attained by casting a silicon melt onto a melt-resistant carrier body having periodically spaced-apart holes or a special mesh structure, through which crystallization seeds for the formation of the desired columnar structures can be positioned or attained.